The present invention generally relates to power system protection, and more specifically, to an apparatus and method for identifying a loss of a current transformer signal in a power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Protective devices and procedures act to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Logically grouped zones of protection, or protection zones utilizing the protective devices and procedures, are established to efficiently manage faults or other abnormal conditions occurring in the power system elements.
In general, protection zones may be classified into six types including: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosers.
Although the fundaments of power system protection are similar, each of the six types of protection zones use protective devices that are based on the characteristics of the power system elements in that category. More specifically, different protective relays utilizing a variety of protective schemes (e.g., differential current comparisons, magnitude comparisons, frequency sensing), are required to protect the various power system elements. For example, a line current differential relay, having nn electrical connections, is designed to monitor current flowing into an overhead transmission line (i.e., a line protection zone) by measuring the current flowing into the overhead transmission line and calculating inter alia, the sum of all measured current, or the operate current. As is known, when the overhead transmission line is operating under normal conditions, the sum of all of the (primary) currents entering the overhead transmission line is about zero (Kirchhoffs current law). If the overhead transmission line has a short circuit, or is faulted, its operate current will be substantially different from zero, indicating that there is some impermissible path through which a current is flowing. If the operate current exceeds some threshold, or pickup current, the line current differential relay issues a tripping signal to an associated power circuit breaker(s) causing it to open and isolate the faulted overhead transmission line from the remainder of the power system.
Because power system currents can easily exceed 10,000 amperes (amps) and power system voltages can reach several thousand volts, and because a protective device, such as the line current differential relay described above, is designed to measure currents no greater than 100 amps via its nn electrical connections, the protective device is coupled to the power system element(s) via a number of current transformers. The current transformers operate to proportionally step-down the power system current flowing into the protection zone (while retaining the same phase relation), to a magnitude that can be readily monitored and measured by the protective device. As is known, the three-phase current flowing into the protection zone is commonly referred to as a primary current, and the current flowing from the current transformers to the protective device is commonly referred to as a secondary current. The resulting lower secondary currents or “CT signals” can be filtered, sampled, etc., by the protective device to determine corresponding phasors representative of the primary current flowing into the protected power system element. The phasors are then used in the various overcurrent, directional, distance, and differential logic schemes of the various protective devices.
Portions of the protection zones typically overlap each other to provide redundancy and to ensure that faults and their locations are properly identified. Thus, each protection zone normally includes protective relays that provide backup for the relays protecting the power system elements of adjacent protection zones. The overlap is typically accomplished via the location of the current transformers. As a result, some of the power system elements and current transformers may be “switched” in and out, or reassigned to, different overlapping protection zones. Such re-assignment may be the result of a change in the power system load, power system reconfiguration, and the like. A description of a system for protection zone selection is provided in U.S. Pat. No. 6,411,865, issued Jun. 25, 2002, by Qin, et al., entitled “System for Protection Zone Selection in Microprocessor-Based Relays in an Electronic Power System,” which is incorporated herein in its entirety and for all purposes.
Due to their integral role in the power system, if a defective current transformer delivers an incorrect or errant secondary current to a protective device (e.g., a current differential relay, an overcurrent distance relay), problems may arise in relay operation. Because the incorrect or errant secondary current is not reflective of the actual operate current, a circuit breaker in the protection zone may fail to trip in the event of a short circuit in the protection zone, or an erroneous trip may occur when no short circuit exists. In other words, the protective device may incorrectly “perceive” a short circuit or other fault in the protection zone when the errant current is actually due to a current transformer problem.
Thus, in some cases when one of the current transformer connections nn between the current transformer and the protective device becomes open or short circuited, the CT signal entering the protective device decreases to substantially zero. In such cases, the protective device can potentially misoperate. For example, in a current differential relay, the missing current creates a false “high” operate current that may potentially exceed the trip threshold. As a result, an unwanted trip signal may be issued, despite the absence of a short circuit in the protection zone. Such an open current transformer connection, or a lost current transformer signal, occurring between the current transformer and the protective device is referred to herein as a lost CT signal or an open CT condition.
Various prior art algorithms have attempted to detect when a lost CT signal is present, however all have limitations. For example, in one prior art algorithm implemented in a current differential relay, a lost CT signal is detected only for CTs carrying an incoming current. Further, while a lost CT signal may be detected using such a prior art algorithm, the specific lost CT signal from among a number of CT signals is not identified. As a result, its associated specific current transformer can not be identified.